Japanese Unexamined Patent Application Publication No. 2012-70614 (Patent Document 1) discloses a wireless power transmission system for transmitting electric power to a power receiving device using an electric field coupling technique. Power is transmitted by electric field coupling between an electrode of the power transmission device and an electrode of the power receiving device. The driving frequency of power transmission is set to enhance power transmission efficiency. This wireless power transmission system performs a frequency sweep to search for a peak resonant frequency (i.e., the resonant frequency at which the driving voltage is at a peak value) when the power receiving device is placed on the power transmission device and performs power transmission to the power receiving device at that resonant frequency.